(1) List of the Prior Art
Japanese Patent Publication No. 10-327193 published on Dec. 8, 1998
(2) Brief Explanation of the Prior Art
In order to secure packets transmitted from inside or outside of a private network through an external network, i.e., the Internet, it has been proposed a virtual private network (VPN) system which provides a secure connection tunnel between the private network and a remote client machine on the Internet. The VPN system includes a VPN controller or VPN server within the private network and a VPN client software running on the client machine for encrypting original packets before transmitting it to the Internet and decrypting the packets after they reach the private network and the client machine. The encryption is accomplished by a security protocol such as Internet Protocol Security (IPSec). The VPN system is given a function of monitoring the encrypted packets transmitted through the Internet for providing a report data including statistical information about the number of VPN tunnels and throughput as well as negotiation information between the server and the client machine. In this connection, Japanese Patent Publication No. 10-327193 discloses a VPN system provided with a gateway that analyzes a header of the packet to determine whether the packets are to be encrypted, decrypted, or required no encryption, and selects a suitable algorithm for encryption and decryption of the packet.
Although the prior VPN system monitors the information about encrypted packets, it fails to check whether or not the packets is truly or successfully encrypted, and therefore may pose a danger of transmitting original unencrypted packets to the Internet.